The present invention relates to the polishing of semiconductor substrates and, more particularly, to the polishing of planar surfaces of silicon, for example.
Semiconductor devices such as integrated monolithic circuits, diodes, passive devices and the like, are formed by various techniques such as diffusion and epitaxial growth, in the planar surfaces of semiconductor substrates For such semiconductor device fabrication, polished silicon wafers, free of undesirable crystal defects and surface imperfections is a basic requirement. Polishing and cleaning of the wafers is normally the last process in the preparation of the wafers for device fabrication.
Although the requirement for a high degree of polishing has heretofore existed in the semiconductor art, the higher levels of microminiaturization of device size, the perfection of surface quality of the polished wafer has become of paramount importance in regard to the feature of surface fine-structure conditions down to an order of Angstrom units, planarity, uniformity and freedom of surface irregularities and crystal damage.
A substantial portion of commercially available silicon wafers are polished with compositions comprised of silicon dioxide having particle sizes in the colloidal range of 1-500 m.mu. (millimicrons) with particles suitable for polishing ranging in size from 10-150 m.mu.. Properly sized silicon dioxide compositions with nominal particle sizes of 16 m.mu. are commercially available, normally in the form of sols and gels.
Virtually any type of polishing cloth, such as synthetic leather, resin non-woven types, can be used with such a silicon dioxide polishing composition, typical of which are Corofam and Blue Rodell. Polishing may be effected in machines such as described in Geotz et al, U.S. Pat. No. 3,691,694 including modification thereof.
During polishing, wheel speeds may vary from 40 to 200 rpm applied at pressures of 2-6 psi. The slurry feed (e.g., SiO.sub.2 polishing composition) is usually continuous at a flow rate of not less than 10 cc per minute and the slurry may be recirculated. The rate of stock removal, from the silicon wafers, is approximately 0.75 mils per hour.
Although the foregoing produces polished wafers having a high and acceptable degree of perfection, the process is nevertheless relatively slow with considerable time required to attain such a degree of perfection. Attempts to accelerate such polishing, as by increasing pressures or nature of abrasives (e.g., size and/or hardness) can result in sub-surface damage which becomes evident in subsequent device fabrication.